A porous film is used for a variety of applications such as various separators for batteries and electrolytic capacitors, various separation membranes (filters), absorbent goods as typified by diapers and feminine hygiene products, waterproof moisture-permeable members for clothing and medical purposes, members for thermal receiving paper, and ink receptor members. A porous film based on polyolefin as typified by polypropylene and polyethylene is mainly used. The porous polyolefin film is used particularly as a separator for an electric storage device because it has features such as high permeability and high porosity.
The electric storage device is one of very important electrical devices that support the current ubiquitous society because it is capable of producing electrical energy anytime and anywhere. Meanwhile, the spread of portable devices such as video cameras, personal computers, cellular phones, portable music players, and handheld game consoles, is increasing the need for an increase in capacity and reduction in size and weight of the electric storage device (particularly, of a secondary battery). A lithium-ion battery, among others, has higher energy density and higher output density both per volume and per mass than those of other electric storage devices, so that the demand for the lithium-ion battery as an electric storage device satisfying the above mentioned need is greatly increasing.
Furthermore, in recent years, there have arisen problems such as global warming, air pollution, exhaustion of petroleum, and CO2 emission control, and an environmental load of automobiles is becoming a large problem. Therefore, studies for development and practical application are being actively carried out regarding electric vehicles (EVs), hybrid electric vehicles (HEVs), fuel-cell vehicles (FCVs), and the like that can be some of solutions for environmental measures (improvement of cleanliness), energy saving measures (improvement in fuel economy), next generation fuel measures (new energy development), and the like. Emphases have been put on, for example, the lithium-ion battery and an electric double layer capacitor as main power sources or auxiliary power sources of these vehicles, and application thereof is being studied at a rapid pace.
In the case of using the porous film as a separator for an electrolytic device, particularly for the lithium-ion battery, one of the requirements is that the porous film needs to have a high thickness recovery rate when a load is applied thereto and released therefrom. A negative electrode of the lithium-ion battery expands and contracts in the thickness direction of the separator each time lithium is stored and released during charging and discharging. The separator needs to follow the expansion and contraction of the negative electrode and change in thickness, and when the separator that is compressed during charging does not recover the original thickness during discharging, there have been cases in which resistance increases, or a short circuit is likely to occur due to particles that have come off or the like.
In some cases, the separator is required to have a large thickness change rate to follow the expansion and contraction of the negative electrode. For example, when the lithium-ion battery uses an alloy-based negative electrode that is expected to have high energy density, but has particularly large expansion and contraction values, there has been a problem that a small thickness change rate leaves no room for the expansion of the negative electrode and thus lowers the battery performance.
Moreover, the separator that follows the negative electrode having the large expansion and contraction values needs to maintain the high thickness recovery rate during any number of times of repetition of charging and discharging. There has been a possibility of occurrence of a problem that a reduction in the thickness recovery rate of the separator with each repetition of charging and discharging gradually increases the resistance relative to an initial value, in other words, deteriorates the cycle characteristics.
Various methods have been developed to form pores of the polyolefin-based film used as the porous film. The methods of forming pores are broadly classified into a wet method and a dry method. As the wet method, a method has been developed (for example, refer to Japanese Laid-open Patent Publication Nos. 55-131028 and 2003-231772) that involves using polyolefin as a matrix resin, adding and mixing a substance to be extracted after forming the resin into a sheet, and then extracting only the substance thus added using a good solvent of the substance to generate pores in the matrix resin. However, the porous film obtained with this method has a three-dimensionally uniform matrix structure, and thus has a high strength in the thickness direction and a low thickness change rate. That is, there has been a possibility of hampering the expansion of the negative electrode during charging and discharging and thus of lowering the battery performance.
As the dry method, for example, a method (what is termed a lamellar stretching method) has been developed (for example, refer to Japanese Examined Patent Publication No. 55-32531 and Japanese Laid-open Patent Publication No. 2005-56851) that involves employing low-temperature extrusion and a high draft ratio during melt extrusion to control a lamellar structure, before stretching, in the film that is formed as a sheet, uniaxially stretching the sheet to generate cleavage at a lamellar interface to form pores. However, the porous film obtained with that method has a structure in which the resin lies in the direction perpendicular to the thickness direction, so that the porous film has a low thickness change rate and, thus, there has been a possibility of hampering the expansion of the negative electrode during charging and discharging and thus of lowering the battery performance.
As the dry method, a number of methods have been developed (for example, refer to Japanese Laid-open Patent Publication Nos. 63-199742, 6-100720, 9-255804 and 2008-120931), called β-crystallization, involving using a difference in crystal density and a crystal transition between an α-type crystal (α crystal) and a β-type crystal (β crystal) that are crystal polymorphs of polypropylene to form pores in the film. However, while being easily deformable in the thickness direction, the porous film obtained with that method has a small thickness recovery rate, and has a large change rate in the thickness recovery rate when loading and releasing are repeated several times. Consequently, there have been cases in which the cycle characteristics deteriorate when the porous film is used for the separator.
It could therefore be helpful to provide a porous film that has a high thickness change rate when a load is applied to the porous film and a high thickness recovery rate when operations of loading and releasing are conducted, to provide a separator for an electric storage device that has good cycle characteristics and that can follow expansion and contraction of a negative electrode in a high energy density battery composition, and to provide an electric storage device.